Converting a Canon Extender FD 1.4 to EOS mount

The original Canon "lens converter FD-EOS" was only sold through Canon Professional Service and few were made, so used ones are hard to get and very expensive. It is said to be of very good optical quality, compared to the cheap 3rd party converters that are available now. In order to adapt from the 42mm register of the FD mount to the 44mm of the EOS mount, some optics are needed in the converter, so it also acts as a 1.26 teleconverter, losing 2/3 stops of light. It also protrudes inside the lens, so it can be used only with a few telelenses.

Looking at the image of the original converter, I realized it looks very similar to a Canon 1.4 teleconverter, which also has this protruding element and can be used with only some larger teles:

Both converters have some optics and both of them add some "length" between the EOS mount and the FD lens, so I thought, why not make the length of the FD 1.4 match the EOS register and therefore make a 1.4 EOS-FD converter with good optics inside.

In order to get something that connects to the EOS mount, I bought a (cheap) M42-EOS adaptor. The adaptor is not totally flat on the front, so it was flattened first:

After removing the back of the 1.4 extender, I measured how thick the "ring" that goes between the M42 and the converter front needs to be and finally settled on the magic value of 20.4 mm between the EOS and the FD mount (20.4 mm includes the front part of the 1.4FD and the M42)

The "ring" was made by a friend and connects to the 1.4 FD through the original screw holes. To attach the M42 3 new holes were drilled into the EOS flange of the M42. The little "stopper block" was added to push the "stop down lever" on the FD lens and therefore to allow to stop down the lens which would otherwise be fully open all the time. When mounting the lens, one has to rotate it slightly into the other direction to engage the stop down lever first, but this is easy to do and works fine. (UPDATE: the stopper block was a bit to short and therefore with some lenses the stop down lever "slipped" past it, it is now glued to the back of the 1.4 FD in the same position - works fine and still can be easily removed if needed)

I wanted to leave the 1.4 FD unmodified, but the "tube" on the back side goes too far into the EOS mount and blocks the mirror on both my EOS600 and EOS33, so about 1.5 mm were taken of:

After assembly the new 1.4 FD-EOS converter is ready to be used:

Now put on FD and EOS caps :-)

UPDATE: I got a new M42 to EOS adapter with "AF-Confirm", this is a little IC on the back of the mount that interfaces with the camera and pretends to it that a lens (with fixed aperture 2.0) is attached. This has two benefits:

The camera will try to autofocus and will therefore enable the AF sensors. If you focus correctly the AF sensors will light up and confirm (hence the name) correct focus.
Some EOS cameras do not measure exposure correctly when no AF lens is attached (like EOS5/EOS30, see here), with the "fake" lens stopped down metering is correct.

These AF-Confirm adapter can be had (different mounts converted to EOS) from various eBay sellers, mine cost about $60. I just added the 3 holes to attach it to the "ring".

EOS 30/Elan 7E with FD 300/2.8 attached:

(all images are 640x480, watch them standalone to see them in full size)